1. Field of the Invention
This invention relates to crucibles used in furnaces for formation of single crystal silicon by the Czochralski method and, more specifically, to a flip top cover therefor which acts as a radiation heat shield.
2. Brief Description of the Prior Art
Prior art crucibles for use in furnaces for melting of polycrystalline silicon (polysilicon) therein and then pulling a single crystal of silicon therefrom by the Czochralski method and the like have generally included a graphite crucible exterior with a quartz liner on the interior thereof. The crucible is situated within a stainless steel enclosure with a graphite heater positioned between the crucible and the stainless steel enclosure. In practice, polycrystalline silicon of semiconductor grade or the like are melted in the crucible and a seed crystal, at the end of a rod, is then placed in the melt and extracted therefrom slowly under standard known conditions to form the single crystal of silicon from which wafers are later formed.
Several problems that have existed with such prior art furnaces and crucibles used therein are, first, that the polysilicon, when melted for later growth of the single crystal, loses much of the energy therein by radiation above the melt, thereby requiring a relatively large amount of power for melting the polysilicon in the crucible. Second, since the heat travels slowly through the melt from the heaters exterior to the crucible, the top of the melt, which is exposed to the furnace atmosphere, permits any oxides which are generated to reside on the melt surface, these oxides being vaporized off only by addition of a greater amount of energy to the melt. The increase in energy fed to the melt, in turn, causes an increase in reactivity between the silicon and the quartz liner of the crucible to increase the amount of oxygen in the melt. The increase of heat to the melt also causes impurities to be generated from the graphite which impurities enter the melt and ultimately enter the crystal. This increase in heat also causes impurities, such as aluminum, to enter the silicon melt due to degeneration of the quartz liner. In addition, the increased heat causes the quartz liner in the crucible to sag, thereby causing non-symmetrical heat distribution in the melt and causing single crystal growth to be more difficult.